Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

(cherry picked from commit 68c81c7375)

tests/functional/gc-concurrent.nix

@@ -0,0 +1,30 @@
+with import ./config.nix;
+
+{ lockFifo ? null }:
+
+rec {
+
+  input1 = mkDerivation {
+    name = "dependencies-input-1";
+    buildCommand = "mkdir $out; echo FOO > $out/foo";
+  };
+
+  input2 = mkDerivation {
+    name = "dependencies-input-2";
+    buildCommand = "mkdir $out; echo BAR > $out/bar";
+  };
+
+  test1 = mkDerivation {
+    name = "gc-concurrent";
+    builder = ./gc-concurrent.builder.sh;
+    inherit input1 input2;
+    inherit lockFifo;
+  };
+
+  test2 = mkDerivation {
+    name = "gc-concurrent2";
+    builder = ./gc-concurrent2.builder.sh;
+    inherit input1 input2;
+  };
+
+}